LED lamp modules are used in many applications. Motor vehicles use a number of lamps and light bulbs to signal driver intention, warnings, and other status of the vehicle. The light bulbs may be located inside or external to the vehicle, and are typically inserted into sockets which are electrically coupled to the vehicle power supply in a controlled manner. For example, the light bulb may be used as an external front, side, or rear turn signal indicator. The light bulb can also be used for headlights, tail lights, back-up lights, brake lights, emergency flashers, and the like. Most, if not all, state and local ordinances require external lights on motor vehicles for visibility and safety. In other cases, light bulbs are used to illuminate the instrument panel, interior compartment, open door, footing area outside vehicle, vanity mirror, cargo area, and trunk.
Lighting for various end products has long been embodied as incandescent light bulbs. The light bulbs operate using various power supply sources, some producing DC voltage as found in most vehicles, and others operate from an AC voltage such as the light bulbs typically found in houses and buildings. Most vehicles operate from a 12 volt DC power supply to the vehicle. As a tail light, when the tail light switch is turned to the ON position, the vehicle's 12 volt DC power supply is applied to the tail light bulb to illuminate the filament. The tail light bulb emits an intensity of light. In the case of a brake light, when the driver depresses the brake pedal, the 12 volt DC power supply is applied to the brake light bulb to illuminate the filament. The brake light bulb emits an intensity of light, greater than the intensity of the tail light. The incandescent light bulb is known to consume significant power, burn hot, and have a relatively short lifespan.
More recently, LEDs lamp modules have been used in lieu of the incandescent light bulbs. Examples of the LED lamp modules are found in U.S. Pat. Nos. 6,371,636 and 6,786,625. The LEDs typically operate at voltages between 1.7 and 2.2 volts, and must be able to produce a light intensity suitable for human perception and recognition from a distance. Since light bulbs are typically operated at higher voltages, the current and voltage must be controlled in order to prevent damage to the LEDs. When used in light bulbs, the LEDs are usually arranged in a matrix or array in the lamp module. Generally, all LEDs in the matrix emit light at the same time. The LEDs emit an intensity of light as a function of the current supplied to the LED matrix.
In the prior art LED lamp modules used in dual intensity applications such as brake lights, turn signals and other dual intensity light bulb applications, when the operator turns on the tail lights, pulls the turn signal lever, depresses the brake pedal, or otherwise activates the light circuit, a DC voltage is applied through a power resistor to the LED matrix. The power resistor converts the DC voltage to an appropriate current for driving the LED matrix. The value of the power resistor determines the magnitude of the current flow to the LED matrix and accordingly, the intensity of illumination of the LEDs. The dim circuit signal path will have a first value of resistance between the DC power supply and the LED matrix. The bright circuit signal path will have a second value of resistance between the DC power supply and the LED matrix. The first value of resistance sets a first current level and illuminates the LED matrix with a first intensity corresponding to a dimmer light. The second value of resistance sets a second current level and illuminates the LED matrix with a second intensity corresponding to a brighter light, e.g., a brake light or turn signal light in a vehicle.
The LED lamp module may also have an electronic switching circuit to connect and disconnect the resistor supplied current to the LED matrix. The switching circuit switches on and off at predetermined frequency to permit the LED matrix to flash, for a turn signal function or emergency flasher function.
The LED voltage drop is a function of the temperature of the LED. When power resistors are used for current control, they cannot adapt to changing temperatures. As a result, the LEDs do not always receive an optimal flow of current. The LEDs may be over-driven which will shorten their life, or under-driven which causes them to appear dim. The power resistors also reduce the life span of the LED and may not allow the LED light module to operate at peak performance over the range of temperatures.
Another problem associated with using power resistors in LED matrix lamp modules is that they are unable to adapt to a variation in DC supply voltage. Although sources of power to buildings and vehicle voltages are set to operate at nominal voltages, the actual voltage may vary considerably. The LEDs are more vulnerable to these variations, since they are not designed to function at the higher voltage. The use of power resistors to control LED light intensity is problematic. If the LED lamp module is set up with a power resistor that will protect the LED at a higher voltage, 14 volts, then the LED will appear dimmer than desired if the supply voltage drops, to 12 volts. If the same LED lamp module is set up with power resistors to operate at full brightness at lower voltage, 12 volts, then the LEDs will burn out prematurely if the system voltage is slightly higher, 14 volts. The use of power resistors in LED lamp modules often results in the premature burn out of the LEDs and/or LED light modules that are unable to operate at optimal peak intensities.
LEDs and multi-LED light bulbs are manufactured in such a manner as to require a particular polarity when used in DC circuits such as an automobile lighting and other DC applications, whereas conventional incandescent light bulbs will function regardless of plus or minus (+/−) polarity in the DC circuit. As a result, many of the conventional sockets commonly used do not key the bulb to ensure that proper polarity is assigned when the bulb is inserted. While this is not a problem with the ambipolar incandescent bulbs, it is a problem with existing LED light bulbs causing the bulb to not function if the polarity happens to be reversed.
A need exists for an LED lamp module which provides substantially consistent brightness over variation in the supply voltage, and temperatures. A need also exists to eliminate the ambiguity of polarity in DC sockets so that LED lamp modules will operate in the DC circuit regardless of the polarity of the wiring connected to the socket, and regardless which way the bulb is inserted into the socket. The LED lamp module design needs to provide for mistake proof installation.